Two consecutively transmitted symbols may experience different propagation routes, and therefore different time in transit, between nodes of a wireless network. This may be referred to as channel delay spread. When these symbols overlap at a receiving node, a portion of the received signal may include intersymbol interference (ISI).
Cyclic prefix is employed in orthogonal frequency division multiplexing (OFDM) systems to mitigate the effects of ISI. Since it adds overhead and reduces system throughput, its duration is minimized. When a cyclic prefix is shorter than the maximum delay, ISI may occur across symbols. This happens when the short cyclic prefix is used in typical urban environments, where the short CP is 4.6875 microseconds (μs) and the maximum delay is 5 μs. The ISI is usually caused by the last multipath returns, which have a power 10 decibels (dB) down from the peak return. Although the average signal to interference power ratio (SIR) is about 22 dB on each subcarrier, the effect of ISI is not negligible. When ISI is ignored, the link adaptation predicts a high throughput of the link and selects a high level modulation and code rate. However, the ISI may result in decoding error and the throughput may start to degrade.